The present invention relates to methods for preparing polyglycerols and polyglycerol esters, while minimizing formation of cyclic polyglycerols and polyglycerol esters.
Polyglycerols and polyglycerol esters are well known in the art. They are commercially available as food, cosmetic, and pharmaceutical emulsifiers, and frequently used as textile lubricants, plastic anti-static agents, defoamers, anti-bloom agents for edible coatings, and anti-splattering agents in cooking oil.
Polyglycerols are commonly prepared by mixing glycerol with an alkali metal catalyst, such as sodium or potassium hydroxide, and then heating the mixture to an elevated temperature. This reaction causes the condensation or dehydration of two glycerol molecules xcex1-hydroxyl groups resulting in an ether bond between the glycerol molecules and the release of water. The unreacted xcex1-hydroxy groups remain available to react with the hydroxy groups of additional glycerol molecules and/or other polymerized molecules.
This method produces a mixture of linear, branched, and cyclic polyglycerols. Cyclic polyglycerols, however, often cause degradation of products into which they are incorporated, detrimentally affecting the taste, performance, and odor of the products. Therefore, there have been continuing efforts to find a method of preparing linear polyglycerols which produce little, if any, cyclic polyglycerols. Cyclic polyglycerols generally have significantly lower hydrophilic-lipophilic (HLB""s) than similar linear polyglycerols. As a result, they typically act as emulsion breakers rather than emulsion builders. Also, the presence of cyclic polyglycerols and cyclic polyglycerol esters promotes free polyglycerol in high mono ester products to precipitate out of solution to produce a two phase system. This makes high mono ester products difficult if not impossible to manufacture on a commercial basis. Polyglycerol esters prepared with a low mole ratio of fatty acid to polyglycerol have a tendency not to be homogeneous at reaction temperature. The situation is worse when cyclic polyglycerols are brought into the system.
Harris, U.S. Pat. No. 2,258,892, describes a method for preparing polyglycerol ethers. The method comprises heating glycerol alone or in the presence of a catalyst to form polyglycerol and etherifying the polyglycerol with a particular alcohol, alkyl halide, alkyl sulphate, or mixture thereof to form a polyglycerol ether.
In U.S. Pat. No. 3,637,774, Babayan describes a process for preparing polyglycerols and esters thereof which have good color and little, if any, odor. The process involves the condensation of glycerol in the presence of an alkaline catalyst in an anhydrous medium at a temperature above 100xc2x0 C. After the condensation has been completed, the reaction mixture is allowed to cool. It is then diluted with water and a bleaching agent is added. The temperature of the mixture is maintained below 100xc2x0 C. for a time sufficient to bleach the product.
Many methods for enhancing the color, odor, and taste of polyglycerols and for reducing the concentration of cyclic polyglycerols in polyglycerol mixtures have been developed. For example, in the method described in Japanese Patent Publication No. 1125338, glycerol is condensed in the presence of an alkali catalyst and aluminum oxide or an aluminum oxide-based absorbent. Low-boiling point components, including unreacted glycerol, are then distilled off and the mixture condensed again to yield polyglycerols.
Japanese Patent Publication No. 61238749 discloses a method of preparing polyglycerols with low cyclic polyglycerol content. The method includes condensing glycerol in the presence of an alkali catalyst and an aluminum oxide absorbent.
Japanese Patent Publication No. 2172938 discloses a process for preparing polyglycerols having a low content of low molecular weight substances, including cyclic substances. The process involves condensation of glycerol in the presence of an alkaline catalyst at controlled temperature and pressure conditions.
Seiden et al., U.S. Pat. No. 3,968,169, disclose heating glycerol under reduced pressure in the presence of a catalytic amount of adjuvants capable of promoting the polymerization of glycerol to form polyglycerols. The condensation reaction is terminated by adding a neutralizing agent to the mixture. Unreacted glycerol and cyclic diglycerol are subsequently removed by distillation. The polyglycerol can optionally be esterified with a fatty acid to form a polyglycerol ester.
In Japanese Patent Publication No. 6279342, the content of cyclic polyglycerols in a crude polyglycerol mixture is reduced by first reacting the mixture with epichlorohydrin in the presence of a Lewis acid and then reacting it with an alkaline aqueous solution.
Jakobson et al., U.S. Pat. No. 4,960,953, disclose a process for preparing polyglycerols which are low in cyclic components. The process comprises reacting glycerol, diglycerol, or a high polyglycerol with epichlorohydrin at 90 to 170xc2x0 C. to produce a crude chlorohydrin/ether mixture; adding an amount of a strong base at least substantially equivalent to the organically bound chlorine content of the chlorohydrin/ether mixture; and desalting the mixture and recovering the glycerol, diglycerol, and higher polyglycerol fractions.
Jakobson et al., U.S. Pat. No. 5,243,086, describe a process for the preparation of diglycerol and other polyglycerols substantially free of cyclic glycerol compounds. In this process, isopropylideneglycerol is reacted with xcex1-monochlorohydrin in the presence of at least one alkaline compound to yield monoisopropylidenediglycerol. The reaction mixture containing monoisopropylidenediglycerol is reacted with water in the presence of at least one acidic catalyst to yield diglycerol and other polyglycerols and acetone.
International Patent Publication No. WO 95/16723 disclose polymerizing glycerol, 2,2-dimethyl-1,3-dioxolane-4-methanol, glycidol, or glycerol carbonate in the presence of an anionic clay to form a polymer of glycerol.
There is a continuing need for more cost effective and faster methods of preparing polyglycerols containing little, if any, cyclic polyglycerols.
Applicants have discovered processes for preparing polyglycerols and polyglycerol esters in high yield and with minimal formation of cyclic polyglycerols and cyclic polyglycerol esters. Furthermore, the processes of this invention produce a high proportion of linear polyglycerols and polyglycerol esters. The linear polyglycerols have very desirable physical characteristics, including a clear appearance at melt, a desirable Gardner color, a mild odor, and a bland taste. The polyglycerols and polyglycerol esters prepared by the method of the present invention are well suited for use as cosmetic and food additives. Since few, if any, cyclic polyglycerols and polyglycerol esters are formed by the processes of this invention, costly and time consuming distillation steps to remove such by-products are not required.
More specifically, applicants have discovered that if a calcium containing compound, such as calcium hydroxide, is used in place of potassium or sodium hydroxide during polymerization of glycerol or esterification of polyglycerols, the formation of cyclic polyglycerols is greatly reduced.
One embodiment of the invention is a method of preparing a polyglycerol comprising polymerizing glycerol, polyglycerol, or a mixture thereof in the presence of a calcium containing compound, such as calcium hydroxide. Preferably, the calcium containing compound is present in a catalytically effective amount. Typically, less than about 8% by weight of cyclic polyglycerols are formed, based upon 100% by weight of total polyglycerols.
Another embodiment is a method of preparing a polyglycerol ester by esterifying or transesterifying a polyglycerol in the presence of a calcium containing compound. According to a preferred embodiment, the polyglycerol is esterified by reacting it with a fatty acid or a triglyceride in the presence of a calcium containing compound.
The present invention provides processes for preparing polyglycerols and polyglycerol esters. These processes produce linear polyglycerols and linear polyglycerol esters in high yield, while minimizing formation of cyclic polyglycerols and cyclic polyglycerol esters. Typically, less than about 8% and preferably less than about 5% by weight of cyclic polyglycerols and polyglycerol esters are formed by these processes (based upon 100% total weight of polyglycerols and polyglycerol esters). The polyglycerols and polyglycerol esters prepared by the processes of the present invention are suitable for use as cosmetic and food additives. Polyglycerol esters prepared by the process of the present invention are phase stable, low in odor and color, bland in taste, and relatively cheap to manufacture.
In any identified embodiments, the term xe2x80x9caboutxe2x80x9d means within 50%, preferably within 25%, and more preferably within 10% of a given value or range. Alternatively, the term xe2x80x9caboutxe2x80x9d means within an acceptable standard error of the mean, when considered by one of ordinary skill in the art.
The term xe2x80x9ccalcium containing compoundxe2x80x9d includes any salt containing a calcium cation, such as calcium hydroxide, calcium oxide, calcium carbonate, and mixtures thereof. A preferred calcium containing compound is calcium hydroxide.
The term xe2x80x9cpolyglycerolxe2x80x9d as used herein includes, but is not limited to, diglycerols, triglycerols, tetraglycerols, and higher oligomeric glycerol polyethers.
Preparation of Polyglycerols
The invention includes a process for preparing polyglycerols by reacting glycerol, polyglycerol, or a mixture thereof in the presence of a calcium containing compound to form a solution containing polyglycerol. The glycerol, polyglycerol, or mixture thereof and calcium containing compound are preferably mixed well. Suitable starting polyglycerols include, but are not limited to, diglycerols, triglycerols, tetraglycerols, higher oligomeric glycerol polyethers, and mixtures thereof. Furthermore, reclaimed glycerol obtained by distilling the excess glycerol from a previously prepared polyglycerol can be used as a starting material. Preferably, most, if not all, ions (except for calcium) and ion containing complexes, such as sodium, potassium, magnesium, and ion containing clays, such as zeolite, diatomaceous, and earth, are removed from the reclaimed glycerol prior to its reaction with the calcium containing compound.
The reaction is typically performed at the same process conditions as polymerization with other alkali metals, such as sodium. Generally, the reaction is performed at a temperature of from about 200 to about 240xc2x0 C. and preferably at from about 220 to about 240xc2x0 C. According to a preferred embodiment, the reaction is performed at about 230xc2x0 C. The reaction is broadly performed at a pressure of from about 10 to about 400 mm Hg and preferably from about 100 to about 200 mm Hg. According to a preferred embodiment, the reaction is performed at a pressure of about 150 mm Hg.
In general, the presence of water in the reaction mixture slows down the reaction. Therefore, it is preferable to have little, if any, water present in the reaction mixture during polymerization. The reaction mixture preferably contains less than about 0.5% and more preferably contains less than about 0.2% by weight of water, based upon 100% total weight of reaction mixture. According to a preferred embodiment, a solvent that promotes water removal, such as toluene, is added to the reaction mixture. According to another preferred embodiment, a heavy inert gas, e.g., nitrogen is sparged through the mixture during the reaction.
Generally, the glycerol, polyglycerol, or mixture thereof is reacted with a catalyzing effective amount of the calcium containing compound. The molar ratio of glycerol, polyglycerol, or mixture thereof to calcium containing compound broadly ranges from about 1:0.0002 to about 1:0.005, preferably from about 1:0.0005 to about 1:0.005, and more preferably from about 1:0.001 to about 1:0.002.
When the calcium containing compound is calcium hydroxide, the weight ratio of glycerol, polyglycerol, or mixture thereof to calcium hydroxide preferably ranges from about 1:0.0003 to about 1:0.009, more preferably ranges from about 1:0.0009 to about 1:0.009, and most preferably ranges from about 1:0.002 to about 1:0.004.
Generally, the reaction mixture is substantially free of alkali (group I) metals, such as sodium and potassium. The reaction mixture preferably contains less than about 0.05% and more preferably less than about 0.0005% by weight of alkali (group I) metals, based upon 100% total weight of reaction mixture.
The reaction mixture can be monitored to assess the completion of the reaction, by, for example, gas chromatography and any method of determining hydroxyl value known in the art.
The resulting solution contains polyglycerol and glycerol. The glycerol can be removed by any method known in the art, such as vacuum distillation or steam stripping. The solution preferably contains less than about 8% and more preferably less than about 5% by weight of cyclic polyglycerols, based on 100% weight of total polyglycerols in the solution.
Preparation of Polyglycerol Esters
According to the present invention, a polyglycerol ester is prepared by esterifying polyglcerol with one or more fatty acids or transesterifying polyglycerol with one or more monoglycerides, diglycerides, or triglycerides in the presence of a calcium containing compound. The polyglycerol may be prepared by the method described above or by any other method known in the art.
Suitable fatty acids include, but are not limited to, saturated and unsaturated, linear and branched C6 to C22 fatty acids. Non-limiting examples of suitable saturated fatty acids include lauric acid, stearic acid, isostearic acid, oleic acid, palmitic acid, behenic acid, myristic acid, caprylic acid, capric acid, caproic acid, arachidic acid, and mixtures thereof. Non-limiting examples of suitable unsaturated fatty acids include myristoleic acid, linoleic acid, oleic acid, licaneic acid, ricinoleic acid, eleostearic acid, and erucic acid. Triglycerides that contain any of these fatty acids are also suitable.
Esterification may be performed at the same process conditions as esterification with other alkali bases, such as sodium hydroxide, sodium carbonate, and sodium acetate. Examples of such process conditions include, but are not limited to, those described in U.S. Pat. Nos. 5,585,506, 4,517,360, 5,006,648, 5,071,975, 5,079,355, and 3,963,699.
According to a preferred embodiment, the esterification (or transesterification) is performed by heating the polyglycerol and fatty acid or triglyceride in the presence of the calcium containing compound. The esterification is generally performed at a temperature of from about 160xc2x0 to about 260xc2x0 C. and preferably performed at from about 210 to about 250xc2x0 C. According to a preferred embodiment, the esterification is performed at a temperature of about 230xc2x0 C. The reaction mixture is typically maintained at a pressure of from about 10 to about 760 mm Hg and preferably is maintained at a pressure of from about 150 to about 760 mm Hg. Transesterification takes place under the same conditions with the exception that the preferred reaction temperature is 230xc2x0 to 260xc2x0 C.
The molar ratio of polyglycerol to fatty acid equivalent (or triglyceride) in the reaction mixture broadly ranges from about 1:0.5 to about 1:10. The molar ratio can be varied in order to vary the performance of the polyglycerol ester produced. Generally, the higher the molar ratio of polyglyercol to fatty acid (or triglyceride), the lower the hydrophilic-lipophilic balance (HLB) of the product.
Typically, a catalyzing effective amount of the calcium containing compound is present in the reaction mixture. The molar ratio of polyglycerol to calcium containing compound in the reaction mixture generally ranges from about 1:0.2 to about 1:0.0002, preferably from about 1:0.04 to about 1:0.0008, and more preferably from about 1:0.004 to about 1:0.001.
According to one preferred embodiment, calcium hydroxide is used as the catalyst at a concentration of about from 0.01 to about 5% by weight, based on 100% total weight of the reaction mixture. More preferably, the concentration of calcium hydroxide in the reaction mixture ranges from about 0.01 to about 2% by weight, based on 100% total weight of the reaction mixture. Since the concentration of calcium hydroxide is relatively low, the calcium may be easily filtered out of the mixture and is not likely to precipitate out of the mixture.
Generally, the reaction is performed for about 1 to about 10 hours and preferably for about 2 to about 4 hours. More preferably, the reaction is continued until the reaction mixture is clear and has an Acid Value as measured by American Oil Chemists Society (A.O.C.S.) Official Method Te 1a-64 of less than 2.
Once the reaction is complete, the reaction mixture can be neutralized by any method known in the art, such as with a neutralizing agent. Suitable neutralizing agents include, but are not limited to, phosphoric acid, phosphorous acid, lactic acid, acetic acid, hydrochloric acid and citric acid.
Generally, the reaction mixture is substantially free of alkali (group I) metals, such as sodium and potassium. The reaction mixture preferably contains less than about 0.02% and more preferably less than about 0.0001% by weight of alkali (group I) metals, based upon 100% total weight of reaction mixture.
The resulting solution preferably contains less than about 8% and more preferably less than about 5% by weight of cyclic polyglycerol esters, based on 100% weight of total polyglycerol esters in the solution.
The following examples are intended to describe the present invention without limitation.
Procedure for Determining Polyglycerol Distribution in Mixture Containing Polyglycerols
In the examples below, the distribution of polyglycerols in a solution was determined according to the following procedure. A 10-50 mg sample of polyglycerol was derivatized with 5 ml of silylating reagent (hexamethyldisilazane (HMDS): trimethylchlorosilane (TMCS): pyridine, 3:1:9 from Supelco, Inc. of Bellefonte, Pa.) following Supelco""s recommended procedure. Supelco""s recommended procedure is as follows:
1. Weigh 1-10 mg of sample into a 5 ml reaction vessel. If appropriate, dissolve the sample in an appropriate solvent. If sample is in aqueous solution, evaporate to dryness, then use neat or add solvent;
2. Add excess silylating reagent at full strength or with a solvent; and
3. Allow the mixture to stand until silylation is complete. To determine when derivation
is complete, analyze aliquots of the sample at selected time intervals until no further increase in product peak(s) is observed.
The polyglycerol distribution was determined by gas chromatography using a DB-5HT capillary column (30 mxc3x970.32 mm IDxc3x970.1 xcexcm film thickness), available from J and W Scientific Inc. of Folsom, Calif., and an HP Series 5890 Chromatography equipped with an FID detector. The conditions for the column were as follows: helium carrier gas (2 ml/min); column head pressure (6.9 psig); injector temperature (375xc2x0 C.); detector temperature (375xc2x0 C.); temperature program (100-375xc2x0 C. at 10xc2x0 C./min ramp, 5 min at 100xc2x0 C. and 10 min at 375xc2x0 C.); and a 3 xcexcl injection volume. Identification of the eluting compounds was determined by gas chromatography and mass spectroscopy.
Procedure for Determining Polyglycerol Distribution in a Mixture Containing Polyglycerol Esters
A 2 g sample of polyglycerol ester and 50 ml of 0.8 N ethanolic potassium hydroxide were placed into a 250 ml Erlenmeyer flask and refluxed for 1 hour. While still warm, the solution was neutralized to a Congo red end point with hydrochloric acid. The fatty acid was allowed to settle and approximately 10 ml of the ethanol layer (which contained free polyglycerol) was removed and transferred to a 20 ml scintillation vial, followed by the addition of approximately 10 ml of an aqueous saturated sodium chloride solution. The mixture in the vial was then boiled to remove residual ethanol. The vial was cooled and 3 ml of the solution was removed and filtered through a disposable HPLC filter into a clean 20 ml scintillation vial. Approximately 5 ml of isopropyl alcohol was added to the vial and the resulting mixture was evaporated to dryness. The dried sample, which contained polyglycerol, was then analyzed by the method described above for determining the polyglycerol distribution in a mixture.
Procedure for Determining Acid Value
The acid value of the reaction mixture was determined according to the American Oil Chemists Society (A.O.C.S.) Official Method Te 1a-64.
Procedure for Determining Hydroxyl Value
The hydroxyl value of the reaction mixture was determined according to A.O.C.S. Official Method Cd 13-60.
Procedure for Determining Saponification Value
The saponification value of the reaction mixture was determined according to A.O.C.S. Official Method Cd 3-25.